Axle driving system

ABSTRACT

An axle driving system which houses in a housing thereof a hydrostatic transmission, axles, and a driving gear train for connecting output means of the hydraulic transmission and axles, so as to transmit power from a driving source to the hydrostatic transmission and to change the speed, thereby driving the axles. A first chamber therein contains the hydrostatic transmission and a second chamber therein contains the driving gear train. Both the first and second chambers are independent of each other so as to prevent a foreign object, such as iron powder produced in the driving gear train, from entering the hydrostatic transmission. The system includes an L-like-shaped center section on which the hydrostatic transmission is offset such that an imaginary plane which includes a motor mounting surface passes in proximity to the axis of a pump shaft. The pump shaft is disposed perpendicular to the axles. The motor shaft is disposed in parallel thereto. A hydraulic pump is positioned between the hydraulic motor and the axles, so that the housing for the hydrostatic transmission, axles and driving gear train, is smaller in width to thereby make the system more compact.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of application Ser. No.11/295,430, filed Dec. 7, 2005 now U.S. Pat. No. 7,127,890; which is aContinuation of application Ser. No. 10/800,638, filed Mar. 16, 2004,now U.S. Pat. No. 6,990,808; which is a Continuation of application Ser.No. 10/101,112, filed Mar. 20, 2002, now U.S. Pat. No. 6,715,283; whichis a Continuation of application Ser. No. 09/381,235, filed Sep. 13,1999, now U.S. Pat. No. 6,449,949; which is a national stage of WOApplication No. PCT/US97/03809, filed Mar. 12, 1997, all of which ishereby incorporated in its entirety herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an axle driving system in which ahydrostatic transmission (hereinafter referred to as an “HST”), axlesand a power transmitting mechanism are integrally provided in a housing,and more particularly to an axle driving system in which the width ofthe portion of the housing which houses the HST and power transmittingmechanism is smaller than in conventional systems.

2. Background Art

A conventional axle driving system houses the HST, axles and a drivinggear train for interlocking the HST with the axles in a common housing.The HST is constructed so that a hydraulic pump is disposed on ahorizontal portion of a center section which is L-like-shaped and ahydraulic—motor is disposed on the vertical portion of the same. Thehydraulic motor is positioned to one side of the axle. The hydraulicpump and hydraulic motor are fluidly connected to each other by a closedfluid circuit formed in the center section. The hydraulic pump is drivenby a prime mover provided on the vehicle so as to drive the hydraulicmotor and then the axles through a driving gear train. Such aconstruction is disclosed, for example, in U.S. Pat. Nos. 5,163,293 and5,335,496.

The hydraulic pump and hydraulic motor in the conventional technique,are disposed side-by-side and to one side of the axles. As such, thewidth of the HST is larger which results in the lateral width of thecommon housing for both the pump and motor also being larger.Furthermore, an output shaft of the hydraulic motor extends to one sideof the vehicle to transmit power therefrom to a differential gear unitthrough gears of a driving gear train, so as to drive the axles. Anunused space is formed at a side of the gear train and between the HSTpump and the axles.

Further, when the HST and the driving gear train for driving the axlesby the output shaft of the HST are housed in a common housing, a foreignobject, such as iron powder produced by the driving gear train, mayenter into the HST. This can adversely affect operation of the HST orvarious parts thereof.

BRIEF SUMMARY OF THE INVENTION

The axle driving system of the present invention is constructed so thatthe HST center section is formed in such a manner that the extendedphantom plane of the motor mounting surface of the center section passesin the vicinity of the axis of the pump shaft of the hydraulic pump. Thepump shaft extends substantially perpendicular to the axles. The motorshaft of the hydraulic motor extends substantially in parallel thereto.The hydraulic pump is disposed between the hydraulic motor and theaxles. Hence, the width of the—housing is made smaller so as to becompact in size. The axle driving system, which is smaller in lateralwidth, is provided with a wide swinging space for the running wheels ofthe vehicle and is extremely effective for a vehicle having freelysteerable wheels mounted thereon.

Further, the present invention divides the housing into two separatechambers for housing the HST and for housing a driving gear train andaxles. A partition for dividing the two chambers is provided with an oilfilter so that both chambers can be filled with common oil. Thisimproves the durability of the HST and reduces the manufacturing cost.

The above and other related objects and features of the invention willbe apparent from a reading of the following description of the preferredembodiments including the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 is a partial cross-sectional plan view of a first embodiment ofan axle driving system of the present invention, from which an upperhalf housing is removed;

FIG. 2 is a cross-sectional view looking in the direction of the arrows2—2 in FIG. 1;

FIG. 3 is a cross-sectional view looking in the direction of the arrows3—3 in FIG. 1;

FIG. 4 is a cross-sectional view looking in the direction of the arrows4—4 in FIG. 1;

FIG. 5 is a cross-sectional view looking in the direction of the arrows5—5 in FIG. 1;

FIG. 6 is a cross-sectional view looking in the direction of the arrows6—6 in FIG. 1;

FIG. 7 is a top plan view of a center section of the present invention;

FIG. 8 is a side elevational view of the same;

FIG. 9 is a bottom plan view of the same;

FIG. 10 is a cross-sectional view looking in the direction of the arrows10—10 in FIG. 7;

FIG. 11 is a cross-sectional view looking in the direction of the arrows11—11 in FIG. 8;

FIG. 12 is a cross-sectional view looking in the direction of the arrows12—12 in FIG. 8;

FIG. 13 is a cross-sectional view looking in the direction of the arrows13—13 in FIG. 7;

FIG. 14 is a cross-sectional view looking in the direction of the arrows14—14 in FIG. 7;

FIG. 15 is a cross-sectional rear view of a portion of the presentinvention surrounding a brake operating shaft;

FIG. 16 is a cross-sectional view looking in the direction of the arrows16—16 in FIG. 15;

FIG. 17 is a cross-sectional view looking in the direction of the arrows17—17 in FIG. 15;

FIG. 18 is a perspective view of the brake operating shaft and a biasingmember of the present invention;

FIG. 19 is a plan view of a second embodiment of the axle driving systemof the present invention from which an upper half housing is removed;

FIG. 20 is a cross-sectional view looking in the direction of the arrows20—20 in FIG. 19;

FIG. 21 is a sectional view looking in the direction of the arrows 21—21in FIG. 19;

FIG. 22 is a side view of an alternative embodiment of the centersection of the present invention;

FIG. 23 is cross-sectional view looking in the direction of the arrows23—23 in FIG. 22;

FIG. 24 is a cross-sectional view looking in the direction of the arrows24—24 in FIG. 22; and

FIG. 25 is a cross-sectional view looking in the direction of the arrows25—25 in FIG. 22.

DETAILED DESCRIPTION OF THE INVENTION

Explanation will now be given on the entire construction of an axledriving system according to the present invention in which the housingthereof comprises an upper half housing 1 and a lower half housing 2which are joined together along a horizontal, flat peripheral jointsurface of each half housing. Along the joint surface of the upper andlower half housings is provided bearings for a motor shaft 4 and acounter shaft 26. Axles 7 are disposed in parallel to the joint surfaceof the housing. The bearings for axles 7 are shifted upwardly from thejoint surface and are disposed in upper half housing 1 so as torotatably support axles 7. Axles 7 are differentially coupled with adifferential gear unit 23. Each axle 7 projects outwardly from one endof left and right side walls of the housing, respectively.

The interior of the housing is divided by an inner wall 8 into a firstchamber R1 for housing therein an HST and a second chamber R2 forhousing therein (1) a driving gear train comprising a plurality of gearsfor transmitting power from motor shaft 4 to differential gear unit 23,(2) differential gear unit 23, and (3) axles 7. Inner wall 8 comprises alongitudinal portion which is in—parallel to axles 7 and α perpendicularportion which extends at a right angle to the longitudinal portion ofinner wall 8. Both portions of inner wall 8 are continuously provided sothat first chamber R1 is disposed adjacent to second chamber R2. Innerwall 8 also comprises a vertical wall portion which extends downwardlyfrom the interior of upper half housing 1 toward the joint surface ofthe housing and rising from the interior of second half housing 2 towardthe same. The end surfaces of both the vertical wall portions of innerwall 8 abut against each other when both upper and lower half housings 1and 2 are joined, thereby forming two divided, independent chamberswithin the housing.

The first and second chambers R1 and R2 are filled with lubricating oilwhich is used in common therewith to form an oil sump. As shown in FIG.6, an oiling lid 6 is provided on an upper wall of upper half housing 1above differential gear unit 23 so as to enable operating oil to besupplied through lid 6. As shown in FIG. 5, an oil flow-through port 75is mounted on a wall surface of upper half housing 1 constituting firstchamber R1, so that first chamber R1 and an external reservoir tank 10fluidly communicate with each other through a piping 9 made of a rubberhose or the like so as to enable operating oil in the oil sump to bemaintained at a predetermined amount. The amount can be adjusted byflowing an incremental volume of oil into reservoir tank 10 when thetemperature of the oil rises when the HST is driven.

An oil filter 18 is disposed on inner wall 8 which partitions firstchamber R1 from second chamber R2. In a first embodiment, as shown inFIGS. 1 and 5, oil filter 18 is disposed at the joint surfaces of thevertical portions of inner wall 8 to house therein the HST and rightside axle 7, thereby enabling oil to flow through oil filter 18 betweenfirst chamber R1 and second chamber R2. Accordingly, oil provided in thehousing can be used in common as operating oil for the HST and aslubricating oil for the gears and bearings. Also, when oil flows fromsecond chamber R2 into first chamber R1, harmful foreign objects such asiron powder, flowing into the HST is filtered by oil filter 18.

First chamber R1 is disposed in front of axles 7 and to the side of thegeared transmission for transmitting power from motor shaft 4 todifferential gear unit 23, provided in the housing. A center section 5of the HST is mounted in first chamber R1 and is separate therefrom.Center section 5 is disposed in a manner such that its longitudinaldirection is substantially perpendicular to axles 7. The front portionforms a vertical surface 91 on which a motor mounting surface 41 isformed on which a hydraulic motor is disposed. The rear portion forms ahorizontal surface 90 on which a pump mounting surface 40 is formed onwhich a hydraulic pump is disposed. Accordingly, the hydraulic pump isdisposed between the hydraulic motor and axles 7. A pump shaft 3 issupported vertically in the center of pump mounting surface 40 and ispositioned between the hydraulic motor and axles 7.

The axial piston type hydraulic pump of the present invention includes acylinder block 16 which is rotatably, slidably disposed on pump mountingsurface 40 of center section 5. Pistons 12 are fitted into a pluralityof cylinder bores and move in reciprocation through biasing springs. Amovable swash plate 11 having a thrust bearing 11 a abuts against theheads of pistons 12. At the center of movable swash plate 11 is formedan opening 11 b through which pump shaft 3 perforates. Pump shaft 3 alsoserves as an input shaft and is disposed along the rotational axis ofcylinder block 16 and is not relatively rotatably retained thereto. Theupper end of pump shaft 3 projects outwardly from the upper wall ofupper half housing 1 and fixedly supports an input pulley 43 having acooling fan 44. Input pulley 43 is given power from a prime mover (notshown) of the vehicle to which the axle driving system is mountedthrough a belt transmission mechanism (also not shown).

The piston abutting surface of movable swash plate 11 is desirablyslantingly movable from a horizontal state with respect to therotational axis of cylinder block 16, thereby enabling the amount anddirection of discharged oil from the hydraulic pump to be changed. Therear surface of movable swash plate 11 is convex and the inner surfaceof a lid member 15 fixed to upper half housing 1, which closes anopening in the upper wall, is made concave to match with the convex rearsurface of movable swash plate 11. Movable swash plate 11 is constructedto be of a cradle type which, when slantingly moved, slides while cominginto close contact with the concave surface of upper half housing 1.

In order to slantingly operate movable swash plate 11, as shown in FIGS.1 and 3, a control shaft 35 extending in parallel to axles 7 isrotatably supported on the right side wall of upper half housing 1opposite to the driving gear train for transmitting power todifferential gear unit 23. A control arm 38 is mounted onto one end ofcontrol shaft 35 outwardly extending from the housing. A swinging arm 39is mounted to the other end of the same, inside the housing. Theswinging arm 39 comprises a first arm 39 a and a second arm 39 b whichextend radially from control shaft 35. A projection 39 c is provided atthe utmost end of second arm 39 b, as shown in FIG. 2. Since controlshaft 35 coincides at the axis thereof with the axis of slanting motionof movable swash plate 11, it is possible to directly engage projection39 c with a groove 11 d formed on a side surface of movable swash plate11. In such a construction, when control arm 38 is rotatedlongitudinally of the vehicle body, swinging arm 39 rotateslongitudinally around control shaft 35 so as to enable movable swashplate 11 to be slantingly moved to thereby change the output of thehydraulic pump.

At the utmost end of first arm 39 a, opposite to projection 39 c, isdisposed an engaging pin 39 d. A bush 51 is fitted onto control shaft 35within the housing. A neutral position return spring 31 of the torsioncoil type is fitted onto bush 51. Both ends of neutral position returnspring 31 cross and extend in the direction of first arm 39 a so as toput between both ends an eccentric shaft 33 mounted onto an inside wallof upper half housing 1 and engaging pin 39 a. Accordingly, when controlarm 38 and swinging arm 39 rotate to change the speed of the vehicle,one end of neutral position return spring 31 is moved to widen a gapbetween both ends, but the other end of spring 31 is retained by theeccentric shaft 33, so that control lever 38 is given a biasing force toreturn to a neutral position. When the operating force on control arm 38is released, a restoring force generated at one end of neutral positionreturn spring 31 holds engaging pin 39 d by eccentric shaft 33 in thespecified neutral position. A portion of eccentric shaft 33 extendingoutwardly of the housing is formed into an adjusting screw and eccentricshaft 33 is preferably rotatably shifted therethrough, so that swingingarm 39 shifts around control shaft 35, thereby enabling movable swashplate 11 to be adjusted to put it into an accurate neutral position.

Control arm 38, as shown in FIG. 2, is provided with an arm 38 b forconnecting a shock absorber 73. A vertical arm 38 a connects to a speedchanging member (not shown), such as a lever or a pedal provided on thevehicle, through a link mechanism (not shown) on the vehicle. Arm 38 bis pivotally supported by a movable member of shock absorber 73. Acasing thereof is pivotally mounted onto a support plate 74 fixed to alower surface of an axle housing portion of lower half housing 2. Shockabsorber 73 prevents control arm 38 from abruptly changing speed andalso prevents the speed changing member (not shown) from abruptlyreturning to the neutral position when operating force is released so asto exert a sudden braking action onto the HST. Also, shock absorber 73is positioned somewhat forwardly slanted and extends along the rightside wall of upper half housing 1 straddling axles 7, therebyeffectively utilizing an otherwise unused or dead space surroundingaxles 7.

Pressurized oil discharged from the hydraulic pump is sent to thehydraulic motor through an oil passage in center section 5. Thehydraulic motor is constructed as shown in FIG. 4. In detail, a cylinderblock 17 is rotatably, slidably mounted on motor mounting surface 41formed on vertical surface 91 of center section 5. A plurality ofpistons 13 are movably mounted in reciprocation in a plurality ofcylinder bores in cylinder block 17, through biasing springs. The headsof pistons 13 abut against a fixed swash plate 37 which is fixedlydisposed between upper half housing 1 and lower half housing 2. Motorshaft 4 is not relatively rotatably retained on the rotational axis ofcylinder block 17 and extends substantially horizontally. One end ofmotor shaft 4 is supported in a bearing bore in motor mounting surface41 of center section 5. The other end is supported by a bearing 76 oninner wall 8 formed along the joint surfaces of upper half housing 1 andlower half housing 2. The utmost end of motor shaft 4 enters into secondchamber R2. Bearing 76 is a sealing bearing for partitioning firstchamber R1 from second chamber R2. An 0-ring 77 is disposed between theouter periphery of an outer ring and inner wall 8.

The driving gear train for transmitting power from motor shaft 4 todifferential gear unit 23, as shown in FIGS. 1 and 6, comprises a gear25 fixed onto motor shaft 4 where it enters into second chamber R2, alarger diameter gear 24 supported onto a counter shaft 26 andpermanently engageable with gear 25, a smaller diameter gear 21supported on counter shaft 26 and integrally rotatable with largerdiameter gear 24, and ring gear 22 of differential gear unit 23 which ispermanently engageable with smaller diameter gear 21. Counter shaft 26is disposed in second chamber R2 adjacent to pump shaft 3 andperpendicular thereto. One end of counter shaft 26 is supported by aside wall of the housing at the joint surface of upper half housing 1and lower half housing 2. The other end is supported by inner wall 8 atthe joint surface thereof. The rotational output speed of motor shaft 4is reduced by larger diameter gear 24, smaller diameter gear 21 and ringgear 22 so as to drive axles 7 through differential gear unit 23. Largerdiameter gear 24 on counter shaft 26 is disposed as close as possible tothe outside surface of ring gear 22 and is overlapped axially therewith,thereby reducing the longitudinal length of the housing. In thisembodiment, the HST is disposed to one side of the driving gear train atthe right side thereof. At a further right side thereof is disposed aspeed changing mechanism for the HST. The hydraulic pump thereof ispositioned substantially in the lateral and longitudinal center of thehousing. Differential gear unit 23 is disposed in an enlarged portion ofthe housing.

A brake disc 19 is fixed on the utmost end of motor shaft 4 in secondchamber R2. As shown in FIGS. 1, 15, 16 and 17, a brake pad 29 and awedge shaped member 70 are interposed between the upper portion of thefront surface of brake disc 19 and the inner surface of upper halfhousing 1 and are supported thereto, movable only in the direction ofthe rotational axis of motor shaft 4. In a space surrounded by innerwall 8 and the surface of brake disc 19 opposite to brake pad 29 (at theleft side of brake disc 19 in FIG. 15), a biasing member 72 and a brakeoperating shaft 14 are disposed. Brake operating shaft 14 is verticallydisposed and is rotatably supported by upper half housing 1 and lowerhalf housing 2. The upper end of brake operating shaft 14 projectsupwardly from the housing and has a brake arm 27 fixed thereto. On anoutside surface of an intermediate portion of brake operating shaft 14in the housing is formed a flat cutout 14 a which is D-like-shaped whenviewed in cross-section. Arch-like-biasing-member 72 is fitted intocutout 14 a and is restricted from axial movement by cutout 14 a and isguided at both sides by the inner surface of upper half housing 1 so asto be slidable only axially of motor shaft 4. Accordingly, when brakearm 27 is rotated to the left or to the right, brake operating shaft 14is rotated. One longitudinal end of cutout 14 a pushes the rear surfaceof biasing member 72 and brake disc 19 is interposed between brake pad29 and biasing member 72 to exert a braking action on motor shaft 4.Wedge member 70 abuts at the lower surface thereof against the upper endof an adjusting bolt 71. Adjusting bolt 71 screws into lower halfhousing 2 and projects outwardly from lower half housing 2, therebyscrewably tightening a lock nut at the intermediate portion of bolt 71for locking wedge member 70. Wedge member 70 is raised or lowered in thehousing as adjusting bolt 71 is rotated so as to advance or retract inthe direction of the rotational axis of motor shaft 4. As brake pad 29is worn, the interval between brake pad 29 and brake disc 19 can beproperly maintained by adjusting bolt 71 which is vertically disposed inlower half housing 2.

Next, explanation will be given on the construction of center section 5in accordance with FIGS. 7 through 14. Center section 5 is largerlongitudinally than conventional center sections. Center section 5 hasthree bolt bores 5 h which are open vertically between a front portionof center section 5 and a rear portion thereof. Center section 5 isfixed to upper half housing 1 through bolts. At the center of pumpmounting surface 40 formed on horizontal surface 90 on an upper surfaceof a rear portion of center section 5 is formed a bearing portion so asto enable the lower portion of vertical pump shaft 3 to be rotatablysupported therewith. Pump shaft 3 is perpendicularly disposed withrespect to axles 7. A pair of arcuate ports 40 a and 40 b are open atboth sides of the bearing for supplying and for discharging oil fromcylinder block 16.

At the front portion of horizontal surface 90 is formed a verticalsurface 91, a phantom plane which includes vertical surface 91 crossesnear the longitudinal axis of pump shaft 3. Center section 5 issubstantially L-like-shaped when viewed in cross section. As shown inFIG. 8, a pair of arcuate ports 41 a and 41 b are also vertically openon motor mounting surface 41 formed on front vertical surface 91, sothat oil is adapted to be supplied to or discharged from cylinder block16 through ports 41 a and 41 b. At the center of motor mounting surface41 is provided a bearing for motor shaft 4 which is disposed in parallelto axles 7.

In order to connect arcuate ports 40 a and 40 b on pump mounting surface40 with arcuate ports 41 a and 41 b on motor mounting surface 41, afirst linear oil passage 5 a and a second oil passage 5 b are verticallyand forwardly bored in a thick portion of center section 5 so as toreduce the lateral length of center section 5.

Motor mounting surface 41 is positioned in front of the substantialcenter of pump mounting surface 40 so as not to increase the laterallength of the HST when the hydraulic motor is disposed thereon. A thirdlinear oil passage 5 c crosses and communicates with an intermediateportion of second linear oil passage 5 b. Arcuate port 40 a on pumpmounting surface 40 is, as shown in FIG. 14, made thinner to communicatewith first linear oil passage 5 a. Arcuate port 40 b is made deeper tocommunicate with third linear oil passage 5 c. Arcuate port 41 a at theupper portion of motor mounting surface 41 communicates with firstlinear oil passage 5 a. Arcuate port 41 b at the lower portion of thesame communicates with second linear oil passage 5 b. Second linear oilpassage 5 b communicates with third linear oil passage 5 c, wherebyarcuate ports 40 a, 41 a, 40 b and 41 b communicate to form a closedfluid circuit so as to circulate operating oil between the hydraulicpump and the hydraulic motor.

Check valves 54 and 55 are disposed at the open ends of first linear oilpassage 5 a and second linear oil passage 5 b and are closed with lids64, as shown in FIG. 10. A lid 65 closes the open end of third linearoil passage 5 c. When subjected to pressure, lids 64 and 65 abut againstprojections 2 a and 2 b formed on the inner wall of lower half housing2. A first communication oil passage 5 d is vertically bored in centersection 5 so as to communicate with inlet ports of check valves 54 and55. Oil passage 5 d communicates with a terminal end of a secondcommunication oil passage 5 g which is horizontally bored in centersection 5. A fore end of second communication oil passage 5 gcommunicates with an inlet port 45 a into which discharged oil from acharging pump 45 is guided, as shown in FIG. 12. A plug 66, as shown inFIG. 9, closes the open end of first communication oil passage 5 d.

Charge pump 45, as shown in FIG. 3, comprises a pump casing which hasinternal teeth for retaining the lower end of pump shaft 3 extendingfrom, the horizontal lower surface of center section 5 and externalteeth engageable with the internal teeth and which is brought into closecontact with the horizontal lower surface of center section 5. The pumpcasing is biased upwardly by a spring interposed between the lowersurface of the pump casing and the inner bottom surface of lower halfhousing 2 and serving also as a relief valve for maintaining a specifiedvalue of pressure of oil discharged from charge pump 45 and filled inthe closed fluid circuit. An annular oil filter 56 is disposed betweenthe inner bottom surface of lower half housing 2 and the horizontallower surface of center section 5 in a manner of surrounding charge pump45, thereby filtering operating oil taken therein.

As shown in FIGS. 5, 10 and 13, in order to fill the closed fluidcircuit with operating oil after the axle driving system is assembled,oiling pipes 52 and 53 are disposed on the horizontal lower surface ofcenter section 5. Oiling pipe 52 communicates with the deep portion ofarcuate port 41 a through an oil passage vertically bored from thehorizontal lower surface of center section 5. Oiling pipe 53 directlycommunicates with second linear oil passage 5 b. Oiling pipes 52 and 53are exposed at the lower ends thereof from the lower outer surface oflower half housing 2 and are closed by lids after the closed fluidcircuit is filled with operating oil.

A by-pass operating arm 60, as shown in FIG. 5, is disposed above upperhalf housing 1 so as to open first and second linear oil passages 5 aand 5 b into the oil sump for idling axles 7 when hauling the vehicle.In detail, as shown in FIGS. 1 and 4, by-pass operating arm 60 is fixedat the base thereof to a by-pass shaft 61 vertically, pivotallysupported to an upper wall of upper half housing 1. Bypass shaft 61extends at the lower end thereof toward the surface of center section 5opposite to motor mounting surface 41 and forms a flat surface at theperiphery of the lower portion.

A through bore 5 f (see FIG. 8) is open on motor mounting surface 41 ofcenter section 5 and is slightly above the center thereof and betweenaccurate ports 41 a and 41 b. A push pin 62 is slidably supported bycenter section 5 in the direction of rotation of the axis of cylinderblock 17. Push pin 62 can at one end abut against a rotatably slidablesurface of cylinder block 17 which comes into close contact with motormounting surface 41, and abuts at the other end against flat surface 61a of by-pass lever shaft 61.

When the vehicle is hauled, an operator operates by-pass operating arm60 outside of the housing causing by-pass shaft 61 to rotate. Flatsurface 61 a pushes push pin 62 toward cylinder block 17. Push pin 62releases cylinder block 17 from motor mounting surface 41, and theclosed fluid circuit communicates with the oil sump in the housingthrough arcuate ports 41 a and 41 b, thereby obtaining free rotation ofmotor shaft 4.

Next, explanation will be given on a second embodiment of the presentinvention in accordance with FIGS. 19 through 25, in which similar partshave been given the same reference numerals as used in the descriptionof the first embodiment. In the second embodiment, the center section isformed in two pieces rather than in one piece as is center section 5 inthe first embodiment. In this embodiment, center section 5′ is formed ofa first piece 5′a and a second piece 5′b which are coupled together. Onhorizontal surface 90 of first piece 5′a is formed pump mounting surface40. A pair of kidney-shaped ports 40 a and 40 b are open on pumpmounting surface 40. On a side surface of a vertical portion 91 ofsecond piece 5′b is formed motor mounting surface 40, on which a pair ofkidney-shaped bores 41 a and 41 b are open. Communicating oil passages100 and 101 are bored in first piece 5′a. The terminal ends thereof areopen on the side surface. Inside second piece 5′b are bored oil passages102 and 103 which communicate with the pair of kidney-shaped ports 41 aand 41 b. The terminal ends of the passages 102 and 103 are open on theside surface. Oil passages 100 and 102, 101 and 103 connect with eachother through the joint surfaces when horizontal portion 90 is coupledwith vertical portion 91, thereby forming a closed fluid circuit forcirculating therein operating oil between the hydraulic pump andhydraulic motor.

Center section 5′ is not provided with bolt insertion bores 5 h as shownin the first embodiment, but is sandwiched between upper half housing 1and lower half housing 2 so as to be restrained from vertical andlateral movements, thereby being fixedly, positioned in the housing.

The advantages of a two-piece center section 5′ include that themanufacturing and processing costs and the number of assembly processesare reduced, which reduces the overall cost of the system. Further,fewer parts are required in that bolts for securing the center sectionin the housing are not required.

When oil leaks caused from the closed fluid circuit in center section5′, oil in first chamber R1 is taken into the closed fluid circuitthrough oil filter 56 and check valves (not shown). In this embodiment,control shaft 35 for slantingly. rotating swash plate 11 of thehydraulic pump is vertically and rotatably supported by an upper wall ofupper half housing 1. Such construction for engaging control shaft 35with swash plate 11 is the same as, for example, that described in U.S.Pat. No. 5,495,712 which is incorporated herein by reference thereto inits entirety.

As seen from the above description, the axle driving system of thepresent invention can be applied to drive the axles of a vehicle so asto sufficiently reduce the mounting space thereof. Vehicles on whichthis axle driving system may be used include agricultural workingvehicles such as lawn and garden tractors, and transportation vehicles.

Although several embodiments have been shown and described, they aremerely exemplary of the invention and are not to be constructed aslimiting the scope of the invention which is defined by the appendedclaims.

1. A hydrostatic transmission comprising: a housing including twodivisional housing parts separably joined to each other through a jointsurface; a center section fixed to one of the divisional housing parts,wherein the center section includes a first surface and a second surfaceseparably fixed to each other; a first cylinder block disposed in thehousing, the first cylinder block including opposite end surfaces,wherein the first cylinder block is rotatably fitted at one of theopposite end surfaces thereof to the first surface of the center sectionso that a rotary axis of the first cylinder block is disposedperpendicular to the joint surface of the housing, and wherein pistonsare reciprocally slidably fitted into the first cylinder block andproject outward from the other of the opposite end surfaces of the firstcylinder block; and a second cylinder block disposed in the housing, thesecond cylinder block including opposite end surfaces, wherein thesecond cylinder block is rotatably fitted at one of the opposite endsurfaces thereof to the second surface of the center section so that arotary axis of the second cylinder block is disposed in parallel to thejoint surface of the housing, wherein pistons are reciprocally slidablyfitted into the second cylinder block and project outward from the otherof the opposite end surfaces of the second cylinder block, and whereinat least a part of the first surface of the center section is disposedbetween planes including the respective opposite end surfaces of thesecond cylinder block fitted to the center section.
 2. The hydrostatictransmission according to claim 1, wherein a plane including the jointsurface includes the rotary axis of the second cylinder block.
 3. Ahydrostatic transmission comprising: a housing including two divisionalhousing parts separably joined to each other through a joint surface; acenter section fixed to one of the divisional housing parts, wherein thecenter section includes a pump mounting surface and a motor mountingsurface separably fixed to each other; a pump cylinder block disposed inthe housing, the pump cylinder block including opposite end surfaces,wherein the pump cylinder block is rotatably fitted at one of theopposite end surfaces thereof to the pump mounting surface of the centersection so that a rotary axis of the pump cylinder block is disposedperpendicular to the joint surface of the housing, and wherein pistonsare reciprocally slidably fitted into the pump cylinder block andproject outward from the other of the opposite end surfaces of the pumpcylinder block; and a motor cylinder block disposed in the housing, themotor cylinder block including opposite end surfaces, wherein the motorcylinder block is rotatably fitted at one of the opposite end surfacesthereof to the motor mounting surface of the center section so that arotary axis of the motor cylinder block is disposed in parallel to thejoint surface of the housing, wherein pistons are reciprocally slidablyfitted into the motor cylinder block and project outward from the otherof the opposite end surfaces of the motor cylinder block, and wherein atleast a part of the pump mounting surface of the center section isdisposed between planes including the respective opposite end surfacesof the motor cylinder block fitted to the center section.
 4. Thehydrostatic transmission according to claim 3, wherein a plane includingthe joint surface includes the rotary axis of the motor cylinder block.